Lymphoid dendritic cells are being characterized along several lines: A. Dendritic cells in situ: Dr. Lei Chen and I (J. Cell Biol. In Press) have described the anatomy of dendritic cells in mouse spleen germinal centers emphasizing their differences from macrophages. We have also studied the interaction of immune complexes with dendritic cells and macrophages. B. Dendritic cells in vitro: We have developed a method for maintaining highly enriched cultures of dendritic cells. After several days in vitro they retain their distinctive morphologic features and fail to acquire most markers of other lymphoid cell types. However they appear to exhibit abundant surface Ia antigen. C. Dendritic cells and lymphoblasts: We have obtained highly enriched populations of mouse spleen lymphoblasts (Steinman, et al; J. Exp. Med. In Press) to study the selective association and dendritic cells of stimulated lymphocytes.